The invention relates to an axial setting device especially for loading the friction plates of a friction coupling or for fixing the shear plates of a viscous coupling. Two coupling parts are co-axially supported in a housing and which are rotatable relative to one another. A pump assembly has housing parts that are connected to the one of the coupling parts and whose conveying elements are connected to the other one of the coupling parts in such a way that, when the parts rotate relative to one another, fluid is conveyed in the pump assembly. A reservoir for fluid has a variable volume and a pressure chamber is delimited by an axial piston. A fluid connection is made from the reservoir to the pressure chamber via the pump assembly and a pressure line and a fluid connection is made from the pressure chamber to the reservoir via a bypass line with a solenoid valve.
Axial setting devices of this type are used, for example, for actuating friction couplings with alternately arranged first and second friction plates, wherein first friction plates of the friction coupling are connected in a rotationally fast way to the first one of the coupling parts and wherein second friction plates of the friction coupling are connected in a rotationally fast way to the second one of the coupling parts. The friction plates, in their entirety, at one end, are axially supported on one of the coupling parts. The friction plates, in their entirety, are loaded at their other end by the axial piston of the axial setting device. Axial setting devices of this type are also used in viscous couplings with alternately arranged first and second shear plates wherein first shear plates of the viscous coupling are connected in a rotationally fast way to the first one of the coupling parts, and wherein second shear plates of the viscous coupling are arranged so as to be freely rotatable relative to the two coupling parts. Clamping rings with a thickness greater than that of the first shear plates are arranged between the second shear plates and connected to the second one of the parts in a rotationally fast way, wherein the second shear plates and the clamping rings, in their entirety can be axially supported at one end on one of the coupling parts, with the second shear plates and the clamping rings, in their entirety, being loadable at their other end by the axial piston of the axial setting device.
Coupling assemblies of this type are used in the drivelines of motor vehicles with one permanently driven axle, with the other axle being driven when required, and are arranged in such a way that they connect the optionally driven axle as a function of the differential speed. This means that if a slip occurs at the permanently driven axle, which slip leads to a speed differential between the two axles, the multi-plate coupling is closed so that the further axle is also driven. A device of this type is known from DE 198 60 532 A1.
It has already been proposed in connection with such a device that fluid conveyed by the pump assembly can be conveyed from the pressure chamber back into the reservoir by a controlled bypass line. However, this only serves to control any excess pressure via an excess pressure valve. An example of such a device is found in DE 195 05 800 A1.
Furthermore, it is known, in principle, that these devices require an externally controllable disconnecting function to be compatible with ABS systems (brake anti-blocking system) or ESP systems (vehicle electronic stability program). Magnetically controllable, mechanical unlocking devices suitable for this purpose are described in DE 39 11 122 C1, for example.
According to the state of the art, disconnection of coupling assemblies of this type in the driveline, as required for ABS and ESP systems, is normally effected by an axial setting device arranged in a stationary housing, which housing is supported against a rotating part of the coupling by means of an axial bearing under load. The disadvantage of such an assembly is that the differential speed of an axial bearing loaded in this way is identical to the speed of the coupling, as a result of which the service life of the entire coupling unit is limited. One example of such an embodiment is the so-called Flaldex Limited Slip Coupling. Another example of the so-called Powr-Lok(copyright) coupling.
It is the object of the invention to provide an axial setting device which achieves the external disconnecting function by simple means. The objective is achieved in that in the fixed housing there is arranged a primary coil which can be excited by external control means, that in one of the parts, there is arranged a secondary coil which can be excited by the primary coil in a contact-free way and that the solenoid valve can be controlled in the opening sense by the secondary coil when excited by the primary coil. The pump assembly can be provided in the form of a gear pump or of a shear pump (Visco Lok(copyright) system) or in the form of an axial piston pump. In an extremely advantageous way, the assembly for actuating the solenoid valve in the bypass line leading to the pump assembly is completely free from friction and, with the exception of the solenoid valve, it is completely free from wear.
In order to ensure that the assembly functions, even in those cases where the parts of the axial setting device and thus, as a rule, also the parts of the coupling assembly, do not rotate, it is necessary to provide the primary coil with an alternating field. If it is assumed that the necessary action is only necessary in the case of a driving speed, it is sufficient to excite the primary coil with DC voltage.
It can be assumed that, normally, the solenoid valve is controlled only in accordance with an opening and closing function. However, without departing from the concept of the invention, it is also conceivable to provide a control valve whose opening cross-section is increased as a function of a differential-speed-dependent control voltage.
The particular advantage of the inventive setting device consists in its extremely spontaneous reaction, which permits a return of the setting in times of less than 20 msec.